1. Field of the Invention
The present invention relates to a bias control circuit, in particular, a bias control circuit for an optical modulator used in an optical transmitter.
2. Background Arts
Technology of digital coherent optical transmission has been developed as the solution for long-distance transmission systems with a large-capacity to overcome recent rapid increase of traffic in communication networks. The technology uses Dual Polarization Quadrature Phase Shift Keying (DP-QPSK), Quadrature Amplitude Modulation (QAM), and the like as modulation and demodulation system. The DP-QPSK system uses two optical beams each having a polarization plane perpendicular to other for multiplexing, and modulates respective optical beams by Quadrature Phase Shift Keying (QPSK).
An optical transmitter for digital coherent optical transmission includes a light source (LD), a multi-level phase modulator, a driver, a bias control circuit, and so on. The light source provides Continuous Wave (CW) light as carrier. The multi-level phase modulator modulates the CW light in response to driving signals. The multi-level phase modulator is, for example, a QPSK modulator for QPSK modulation. The driver provides the driving signals. The bias control circuit provides bias voltages as reference potentials for the driving signals, and maintains the bias voltages in respective optimum values thereof (for example, refer to Patent Literature 1). The modulated signal output from the multi-level phase modulator is transmitted as an optical output signal to the outside through an optical waveguide path.
To drive the multi-level phase modulator under the optimum conditions is essential for modulating an optical signal (CW light) by the multi-level phase modulator. It is known that the multi-level phase modulator inherently shows some very slow changes (drifting phenomenon) of optical power of a modulated signal output from the multi-level phase modulator after the multi-level phase modulator begins modulation, even when the driving signals are maintained in the initial values thereof. Accordingly, the multi-level phase modulators need an Automatic Bias Control (ABC) which detects the drifting phenomenon of optical power and automatically adjusts the bias voltages to respective optimum values thereof to cancel out influence of the drifting phenomenon (refer Patent Literature 1 to 5). For example, Patent Literature 1 and 4 describe ABC circuits for intensity modulation systems, and Patent Literature 2, 3, and 5 describe ABC circuits for Differential Quadrature Phase Shift Keying (DQPSK) systems.    Patent Literature 1: Japanese Patent Application Laid-Open No. H8-248366.    Patent Literature 2: Japanese Patent Application Laid-Open No. 2008-187223.    Patent Literature 3: Japanese Patent Application Laid-Open No. 2008-92172.    Patent Literature 4: Japanese Patent Application Laid-Open No. 2012-257164.    Patent Literature 5: Japanese Patent Application Laid-Open No. 2013-26758.
In the market of optical transmission systems which constitute core networks and interconnections between servers in data centers, there have been consecutive demands for downsizing and power saving so as to realize higher capacity by higher density of communication channels. The downsizing and power saving of optical transmitters also have been required to enhance the high density of communication channels. For example, in the CFP MSA (100 G Form-factor pluggable Multi-source Agreement) for 100 Gbps optical transceivers, formulation of the second generation standards CFP2 targeting half size of CFP and the third generation standards CFP4 targeting quarter size of CFP have been promoted.
In prior arts of ABC, several analog parts like oscillator, amplifier, mixer, and filter are used to constitute a control circuit that generates a dither signal and performs synchronous detection of the dither signal from a monitor signal. Such analog parts, however, are not suitable for compaction of the control circuit, because size and power consumption thereof are difficult to be reduced. Therefore, a compact ABC circuit operable in low power consumption may contribute downsizing and power saving of optical transmitters.